1. Field of the Invention
The invention relates to a light control unit.
2. Description of the Related Art
For high-capacity recording, digital information recording systems using the principle of hologram have been known recently (for example, see Japanese Patent Laid-Open Publication No. 2002-297008).
FIG. 14 is a diagram showing an example of a hologram recording apparatus. The hologram recording apparatus 100 chiefly comprises a laser source 102, a beam splitter 104, a beam expander 106, a spatial light modulator SLM 108, a hologram pattern writing unit 110, a Fourier transform lens 112, a recording medium 114, a mirror 116, and a rotating mirror 118. Here, a display unit of transmission type is used as the spatial light modulator SLM 108.
In the hologram recording apparatus 100, laser light emitted from the laser source 102 is split into two beams by the beam splitter 104. One of the beams is expanded in diameter through the beam expander 106, and irradiates the spatial light modulator SLM 108 as parallel light. The hologram pattern writing unit 110 sends a hologram pattern to the spatial light modulator SLM 108 in the form of an electric signal.
Based on the electric signal received, the spatial light modulator SLM 108 forms a hologram pattern on a plane surface. The light irradiating the spatial light modulator SLM 108 is transmitted through the spatial light modulator SLM 108 for light modulation, thereby being turned into signal light that contains the hologram pattern. This signal light is passed through the Fourier transform lens 112 for Fourier transform, and focused into the recording medium 114.
Meanwhile, the other of the beams split by the beam splitter 104 is introduced via the mirror 116 and the rotating mirror 118 into the recording medium 114 as reference light. In the recording medium 114, the optical path of the signal light containing the hologram pattern and that of the reference light intersect to form an optical interference pattern. The entire optical interference pattern is recorded on the recording medium 114 in the form of changes in refractive index (refractive index lattice).
The hologram recording apparatus 100 records a single frame of image onto the recording medium 114 in this way. When the recording of a single frame of image is completed, the rotating mirror 118 is rotated by a predetermined amount and shifted in position by a predetermined amount in parallel. The angle of incidence of the reference light on the recording medium 114 is thus changed, and the second frame of image is recorded in the same procedure. Such processing is repeated for angular multiplexing recording.
The spatial light modulator SLM of the hologram recording apparatus may be made of materials that have electrooptic effect, such as PLZT. PLZT is a transparent ceramic having the composition of (Pb1-yLay) (Zr1-xTix)O3. The electrooptic effect refers to the phenomenon that a substance causes polarization with a change in its refractive index when an electric field is applied to the substance. Through the use of the electrooptic effect, it is possible to switch the phase of light by turning on/off the applied voltage. The light modulating materials having the electrooptic effect can thus be applied to optical shutters such as the spatial light modulator SLM.
For these applications including optical shutter devices, bulk PLZT has been widely used conventionally (Japanese Patent Laid-Open Publication No. Hei 5-257103). It is difficult, however, for the optical shutters using bulk PLZT to meet demands for finer configuration, higher integration, lower operating voltage, and lower cost. Besides, the bulk method includes mixing metal oxides, or materials, and then processing the mixture at high temperatures of 1000° C. and above. If this method is applied to the device formation processes, a lot of constraints will thus be imposed on the selection of the materials, the device structures, and so on.
Under the circumstances, attempts have been made to apply a thin film of PLZT formed on a base member to light control devices, instead of bulk PLZT. Japanese Patent Laid-Open Publication No. Hei 7-146657 describes a display unit in which a PLZT film is formed on a glass or other transparent substrate, and comb-shaped electrodes are formed thereon. This display unit has the configuration that a display substrate having the PLZT film formed thereon is interposed between polarizing plates. Here, electrode terminals of the individual pixels are connected to an external drive circuit. Desired pixels are then driven so that transmission light from a light source arranged on one side of the display substrate gives desired display.
In order for the light modulating film such as the PLZT film described above to be put into practical use as an optical shutter device or the like, however, it is necessary that a drive circuit for turning on/off the voltage applied to the light modulating film be formed on the substrate along with the light modulating film. In such configurations as described in the aforementioned Japanese Patent Laid-Open Publication No. Hei 7-146657, the area where the drive circuit is formed on thus cannot be used as a display area. This has produced the problem that the effective display area cannot be secured sufficiently. Besides, the display unit of transmission type as described above has also had the problem that if visible light is used as the irradiation light, the drive circuit cannot be formed on the substrate of silicon or the like which is not transparent to the visible light.